Organic coating films for protecting metal containers or metal closures prevent corrosion of the metal substrate due to the contents, etc. Moreover, metal containers undergo machining such as neck-in processing, bead processing, or seaming of closures. Metal closures with an easy open end (may be referred to hereinafter as “EOE”) are also subjected to harsh processing such as scoring or riveting. Thus, excellent corrosion resistance and processability are required of coating compositions used for metal containers or metal closures.
To satisfy the above-mentioned requirements, various polyester-based coating compositions have been proposed.
Patent Document 1 to be indicated later, for example, proposes a coating composition comprising a hydroxyl group-containing polyester resin, a specific phenolic resin crosslinking agent, and an acid catalyst mixed in predetermined amounts. This coating composition is described as being capable of forming a coating film which has a good balance between processability and hardness, and is excellent in adhesion, curability, hygiene and boiling water resistance. Patent Documents 2 and 3 shown below describe coating resin compositions prepared using polyester resins containing molecules having pendant carboxyl groups introduced into the molecular chains. The aqueous resin composition of Patent Document 2 is described as excellent in curability, processability, retort resistance, hygiene, spray coatability and water dispersibility, while the coating resin composition of Patent Document 3 is described as being particularly superior in adhesion to metal surfaces and fulfilling high processability and high corrosion resistance.
Furthermore, polyester-based coating compositions involve the characteristic problem that the processability of the film after coating declines over time during storage.
Patent Document 4 indicated below proposes a polyester resin composition containing a polyester resin having a low Tg and a crosslinking agent, the polyester resin consisting essentially of 2-methyl-1,3-propanediol, 1,4-butanediol and 1,4-cyclohexanedimethanol as a polyalcohol component. This polyester resin composition is described as having processability, retort resistance, flavor properties, resistance to contents, feathering resistance, and sulfur resistance. The polyester resin composition is also described as having high processability and feathering resistance required for an inner surface coating for EOE, and as being capable of suppressing deterioration in processability over time. Patent Document 5 proposes a coating composition with a moderately branched polymeric polyester resin and containing a vertically asymmetric dihydric alcohol component. This coating composition is described as showing no decline over time in the processability of the film coated, and as being excellent in adhesion to materials and having a good balance between hardness and processability.
Patent Document 4 describes that the preferred polyester resin has a glass transition temperature (Tg) of 20-50° C., whereas Patent Document 5 mentions no glass transition temperature.